Channel Subsystem; Multiple Channel Subsystems And Multiple Subchannel Sets - IBM z13s Technical Manual

5.1 Channel subsystem

Channel subsystem (CSS) is a group of facilities that z Systems uses to control I/O
operations.
The channel subsystem directs the flow of information between I/O devices and main storage.
It allows data processing to occur at the same time as I/O processing, which relieves data
processors (CPs, IFLs) of the task of communicating directly with I/O devices.
The channel subsystem includes subchannels, I/O devices that are attached through control
units, and channel paths between the subsystem and control unites.
The design of z Systems offers considerable processing power, memory size, and I/O
connectivity. In support of the larger I/O capability, the CSS structure has been scaled up
correspondingly by introducing the multiple logical channel subsystem (LCSS) with z990, and
multiple subchannel sets (MSS) with z9.
z13s servers are designed to support up to three logical channel subsystems (LCSS), and
three subchannel sets.
Figure 5-1 summarizes the multiple channel subsystems and multiple subchannel sets.
Figure 5-1 Multiple channel subsystems and multiple subchannel sets
All channel subsystems are defined within a single configuration, which is called I/O
configuration data set (IOCDS). The IOCDS is loaded into the hardware system area (HSA)
during a CPC is power-on reset (POR), to initial all the channel subsystems. On z13s servers,
the HSA is pre-allocated in memory with a fixed size of 40 GB, as well as the customer
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IBM z13s Technical Guide